This type of semiconductor mounted board is used for applications requiring high reliance such as automobile apparatuses, computers, etc., and in small portable apparatuses such as PDAs (Personal Digital Assistants), clocks and watches, portable phones, cameras, etc., but it is usually produced by mounting a flip chip or a semiconductor package comprised of a semiconductor element held on a carrier base onto a wiring substrate, for example, by connecting them via bumps or solder balls. However, in the case of portable phones, a bond-failure of the bumps or the solder balls may occur by the deformation of the substrate caused by falling shock, external pressure generated with the operation of buttons, etc., and hence, a reinforcing measure is employed by filling the gaps between the bonded parts with an underfilling material and curing it to encapsulate them.
In the meantime, when oxide is formed on a solder surface, it gives adverse effects on the bonding force, and hence, a method is usually employed by applying an alcohol solution of a flux (a pine resin component consisting of abietic acid) onto the bonded surface to remove the oxide, and connecting with solder balls, followed by washing and removing the applied flux before the application of an underfilling material.
Thus, the fluxing process is adopted for improvement of the bonding force of bumps or solder balls, but it requires two steps (flux applying and cleaning or removing steps.
On the other hand, in the prior art a thermosetting type epoxy resin composition is used as the aforesaid underfilling material, but if an acid anhydride commonly used as a curing agent is used, it requires the aforesaid step of fluxing process and moreover, when the temperature goes high (to reflow temperature), it has volatilized and may invite generation of voids as a result.